There they are, discreetly lined up on lower shelves. An elegant black strip along the bottom, a demure reproduction of an Old Master above. This cover screams "intellectual!" - but in an understated way, of course. Even if you don't read Penguin Classics, you'd like to be seen carrying one.

Then along comes a brash, shouty upstart, bearing a technicolour image - a winsome-looking Hollywood actress perhaps, or a clutch of familiar TV faces in period dress. In place of the black strip are the words: "Now a major motion picture" or "As seen on TV". The quote might be from the Daily Mail. This hoity-toity newcomer - the Nicole Richie of book covers - is only around for a brief period, but boy does she demand attention.

"It's a no-brainer. You'd be crazy not to do it," says Marcella Edwards, senior commissioning editor at Penguin Classics. The sales surges that come with a film or TV tie-in book cover are irrefutable. Keira Knightley, currently adorning Penguin's film edition of Pride and Prejudice has enjoyed "phenomenal sales" according to Edwards. "The film tie-in jacket is very much led by the film image. For example, I've got Oliver Twist in front of me - the Polanski film from last year. The image is very obviously Oliver Twist but it also links entirely with the film, so it's immediately recognisable."

Sales of Truman Capote's In Cold Blood have been flying off the shelves in the wake of Philip Seymour Hoffman's Oscar-winning portrayal of the writer in Capote, though interestingly, Penguin fought shy of putting a still from the film on the book jacket.

"It would have been misleading and disingenuous," says Edwards. Instead they went for a sticker, linking the book with the film. "Because the film is about the writing of In Cold Blood, it made people want to buy the book."

The film or TV tie-in cover, which generally lasts for around three months (the life of the film, and sometimes the DVD), often running alongside the original paperback design, is an ever-growing trend in publishing. "It's happening more and more often," says Edwards. "Publishers have got wiser. You'd be stupid if you didn't do it."

She points to the sales "spike" for a classic such as Bleak House, which usually chugs along nicely with its understated Black Classic cover, enjoying sales of around 20,000 a year in the UK. With the blockbuster BBC adaptation, the Black Classic edition suddenly sold eight times the volume compared with the same period the previous year. The tie-in jacket, showing a full cast, including Gillian Anderson as Lady Dedlock, shifted an additional 50,000 copies all by itself.

"There is an approachability the film taps into," says Edwards. "It's reaching a new market inasmuch as it becomes less classic, less difficult. You don't need a PhD to read this stuff - it's readable."

The exception that proves the rule is the series of Harry Potter book covers. Bloomsbury has never swapped the original, retro-looking Potter cartoons for film stills showing child star Daniel Radcliffe (apart from on some minor merchandising). The publisher declines to talk about anything Potter-related but there's little doubt about why no tie-in is needed. JK Rowling's books are bigger and more powerful than any film adaptation. She doesn't need the help.

Book designer Miriam Rosenbloom is about to see her much-loved cover design for Santaram, by Gregory David Roberts, bumped in favour of a still showing Johnny Depp in the forthcoming film adaptation. "They want people to make the connection with the big-screen adaptation," she sighs, sounding resigned. "It will go to tie-in and then get re-done."

Rosenbloom, an award-winning designer in her native Australia, is now working in London and has recently experienced the sea change of moving from literary fiction to designing mass-market covers at Transworld. "You have to be more aware of the market," she says. "When you're doing a thriller, you research similar titles so you know what you're competing with. There's a lot of embossing, silver foil, all caps sans serif. It's a strong look, so someone can glance at the cover and unconsciously know what sort of book it is, without reading the blurb. With a women's fiction book, the brief will tell you what the similar titles are. So they will say: this is competing against Freya North and Maeve Binchy; this will sell in supermarkets; women need to want to pick this up."

This contrasts with the design process for literary fiction, where covers are destined for outlets such as Waterstone's, rather than supermarkets. Rosenbloom has worked on numerous titles for Profile Books (which had an unexpected bestseller in Lynne Truss's Eats, Shoots & Leaves), including Christopher Rush's critically acclaimed memoir To Travel Hopefully, a book about his wife's death and his struggle to overcome his grief.

"The book is good writing that's accessible," says Rosenbloom. "It's Waterstones, not supermarket, so you don't have to make the type work as hard; you can go with a quieter-looking cover. After I'd read the book, I was really struck by the imagery of the Cevennes mountains [where the author travels, in the footsteps of Robert Louis Stevenson], so I really wanted to hunt that down. I got the image from a French woman who was the only person I could find with photos of the Cevennes.

"There was a general feeling that we wanted the cover to be quite hopeful, without being overbearingly so. It's a sad book, but it's not impossible to read. We played about with having an image of a man silhouetted above the mountains - having a human element - but we ended up feeling that a stillness worked better."

The human element is something that has disappeared from covers more and more of late, with the trend for freehand lettering and graphic illustration rather than literal photographic images. It began with Jonathan Safran Foer's Everything Is Illuminated, and was soon adopted across the board, on everything from Nick Hornby's A Long Way Down, to Rip It Up and Start Again by Simon Reynolds. Rosenbloom associates it with a passion for all things retro in current book design - "the kind of thing you would have seen in the 1940s or 50s".

"There's A Short History of Tractors in Ukrainian, by Marina Lewycka, which looks like a children's Russian picture book from the 1920s. If you walk into any bookshop at the moment, the quality of the designs are so amazing that your cover has to work really hard."

For this we should be thankful. Film tie-in covers might be glossy and glittering and force a surge in sales, but they are truly the Ivana Trumps of the book jacket world. For proof, you need only glance at the motion picture tie-in for Captain Corelli's Mandolin - a truly saccharine, fog-focus number showing Nicholas Cage eating Penelope Cruz's face on the banks of the Med. It wasn't long before the book's original, and iconic, paperback cover showing a Greek-ceramic-like white and blue illustration, was back in the top slot.